page_title: Hydropneumatic suspension
section_title: Working fluid
hierarchical_section_title: Hydropneumatic suspension / Working fluid
caption_attribution_description: veerbol Xantia
context_page_description: Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce, Maserati and Peugeot. It was also used on Berliet trucks and has more recently been used on Mercedes-Benz cars, where it is known as Active Body Control. The Toyota Soarer UZZ32 "Limited" was fitted with a fully integrated four-wheel steering and a complex, computer-controlled hydraulic Toyota Active Control Suspension in 1991. Similar systems are also widely used on modern tanks and other large military vehicles. The suspension was referred to as oléopneumatique in early literature, pointing to oil and air as its main components.
  The purpose of this system is to provide a sensitive, dynamic and high-capacity suspension that offers superior ride quality on a variety of surfaces.
  A hydropneumatic system combines the advantages of two technological principles:
  Hydraulic systems use torque multiplication in an easy way, independent of the distance between the input and output, without the need for mechanical gears or levers.
context_section_description: Citroën quickly realized that standard brake fluid was not ideally suited to high pressure hydraulics, and developed a special red-coloured hydraulic fluid named LHS (Liquide Hydraulique Synthétique), which they used from 1954 to 1967. The chief problem with LHS was that it absorbed moisture and dust from the air, which caused corrosion in the system. Most hydraulic brake systems are sealed from the outside air by a rubber diaphragm in the reservoir filler cap, but the Citroën system had to be vented to allow the fluid level in the reservoir to rise and fall, thus it was not hermetically sealed. Consequently, each time the suspension would rise, the fluid level in the reservoir dropped, drawing in fresh moisture-laden air. The large surface of the fluid in the reservoir readily absorbed moisture. Since the system recirculates fluid continually through the reservoir, all the fluid was repeatedly exposed to the air and its moisture content.
  To overcome these shortcomings of LHS, Citroën developed a new green fluid, LHM (Liquide Hydraulique Minéral).  LHM is a mineral oil, quite close to automatic transmission fluid. Mineral oil is hydrophobic, unlike standard brake fluid; therefore, water-vapour bubbles do not form in the system, as would be the case with standard brake fluid, creating a "spongy" brake feel. Use of mineral oil has thus spread beyond Citroën, Rolls-Royce, Peugeot, and Mercedes-Benz, to include Jaguar, Audi, and BMW.
  LHM, being a mineral oil, absorbs only an infinitesimal proportion of moisture, plus it contains corrosion inhibitors. The dust inhalation problem continued, so a filter assembly was fitted into the hydraulic reservoir. Cleaning the filters and changing the fluid at the recommended intervals removes most dust and wear particles from the system, ensuring the longevity of the system. Failure to keep the oil clean is the main cause of problems. It is also imperative to always use the correct fluid for the system; the two types of fluids and their associated system components are not interchangeable. If the wrong type of fluid is used, the system must be drained and rinsed with Hydraflush (Total's Hydraurincage), before draining again and filling with the correct fluid. These procedures are clearly described in DIY manuals obtainable from automotive retailers.
  The latest Citroën cars with Hydractive 3 suspension have a new orange coloured LDS hydraulic fluid. This lasts longer and requires less frequent attention. It conforms to DIN 51524-3 for HVLP.
caption: LHM reservoir and green suspension sphere in a Citroën Xantia
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